This invention relates to a differential pressure transmitter and more particularly to a differential pressure transmitter having a suitable structure for preventing damage to differential pressure detecting diaphragms even when a differential pressure to be measured between two front and rear points by orifice plates changes abruptly and substantially.
The structure of a conventional differential pressure transmitter of this kind will be explained. When a differential pressure .DELTA.P is applied, it is transmitted to the front and reverse of a measuring diaphragm equipped with a pressure sensing element such as a semiconductor pressure sensor and is converted to an electric signal in proportion to .DELTA.P, and its output is sent to an external device.
When the differential pressure .DELTA.P becomes great, the output of the pressure sensing element also increases. When .DELTA.P becomes even greater, a pressure receiving diaphragm is seated on a wave-shaped portion of a body. Therefore, when .DELTA.P becomes somewhat great, it stops at a predetermined value as shown in FIG. 4(a) and the output .DELTA.E of the pressure sensing element reaches a certain saturation output .DELTA.E'. this mechanism is ordinarily referred to as a "protection mechanism" for preventing damage to a measuring diaphragm even when an excessive differential pressure exceeding a measurement range is applied to the pressure sensing element in the differential pressure transmission passage.
As the prior art reference associated with this field of art, mention can be made of U.S. Pat. No. 4,713,969 and Japanese Patent Laid-Open Nos. 60-237337/1985 and 60-238732/1985.
However, the protection mechanism operation described above occurs when the pressure to be measured is applied statically. In an actual plant, however, there is the case where the pressure changes abruptly or drastically or is applied drastically. Accordingly, the measuring diaphragm equipped with the pressure sensing element must be provided with a protection mechanism which prevents a differential pressure exceeding an allowable range from being applied to the diaphragm even in a case of abrupt or drastic pressure changes or application. For example, if a cut-off valve is operated depending on any conditions of the plant during the measurement of the differential pressure and the measurement flow rate is stopped drastically, a drastic pressure rise (which is an impact pressure generally referred to as "water hammer" and "steam hammer") takes place as represented by .DELTA.P in FIG. 4(b) in the measurement plant, and this pressure is applied to the differential pressure transmitter and can eventually damage the measuring diaphragm equipped with the pressure sensing element.
As described above, the prior art considered only the over-load protection mechanism when the pressure to be measured changes statically, or slowly but did not take into consideration a means for preventing damage to the measuring diaphragm equipped with the sensing element when the pressure to be measured rises or changes drastically or abruptly. Accordingly, there has been the problem in that the measuring diaphragm of known differential pressure transmitters is damaged by a transient impact pressure.